Only children have never had to accommodate for other people: It's not like we had little brothers or sisters to wait on. I had two dogs at the time, so I said, "Yes, I have two: Baxter and Charlie."Then, a bully by the name of Cathy felt the need to interrupt me and tell me that pets can't be family. Regardless, however, I remain firm that my dogs are my siblings.
As such, it takes some time for me -- for us -- to get used to sharing a bed.
Accommodating for another person in a bed that formerly housed just one person is a transition.
Since there is more money available to those with fewer (or no) siblings, our parents are able to spend more money on our interests and hobbies."Alone time" was yet another thing we were used to having growing up. So, I'm sorry, but there are some days when I just want everybody around me to (pardon my French) fuck right off. If you don't do so when you're asked, you're in for a whole lot of hurt (to put it lightly). So, we might be reluctant to do so even when we're in a relationship.
We're not being rude or selfish; it's just how we were raised.